Ger. Pat. 3538619 discloses the use of cis-4-hydroxy-L-proline in the treatment of carcinomas and related tumors. Eur. AS B 0223850 discloses the use of N-methyl-cis-4-hydroxy-L-proline and N-methyl-trans-4-hydroxy-L-proline in treating astrocytoma cells. Said Eur AS refers to various other configurations, OH-positions, and alkylations, of the said derivatives of hydroxyproline and proline. W. Hoerrmann et al., 1987, Abstract No. 2:046, "Differential effects of cis-4-hydroxy-L-proline and methyl-cis-4-hydroxy-L-proline on tumour cells--evidence for redifferentiation and cell growth inhibition", in Cancer detection and prevention, 11, (1-2):66.